A coalition of environmental groups wants air-quality officials to determine if the refining of heavy Canadian crude has an effect on local health and safety.

A coalition of environmental groups says it has discovered that large-scale shipments of low-quality heavy crude oil from Canada's tar sands are being delivered by rail for processing by Southern California refineries.

The groups on Tuesday called for an investigation by air-quality officials to evaluate the effects on health, air quality, safety and the climate of processing the heavy Canadian crude, which requires intensive processing to remove higher levels of sulfur to meet U.S. standards.

The Natural Resources Defense Council and Communities for a Better Environment say they worry that refineries now processing the semi-solid form of oil have increased their noxious emissions and raised risks of accidental spills and accidents. The process of refining tar sands oil is more corrosive on refinery equipment and produces more greenhouse gases than liquid crude, environmentalists said.

"Tar sands crude is a whole new level of bad," said Julia May, senior scientist at the Communities for a Better Environment, a nonprofit dedicated to preventing and reducing pollution in California's low-income communities. "Bringing it into the Los Angeles area by rail has taken everyone by surprise."

Of particular concern is the low-income community of Wilmington, a Los Angeles harbor town surrounded by five oil refineries and long decried by social justice groups as a "sacrifice zone" of commerce and toxic pollution. Three of the Wilmington refineries — Valero Energy Corp., Phillips 66 Co. and Tesoro Corp — recently announced plans to use rail cars to bring in more of the heavy Canadian crude.

Joe Gorder, president and chief executive of Valero Energy Corp., told shareholders recently that his company plans to import an additional 30,000 barrels a day of the Canadian crude to its Wilmington refinery. Deliveries of the heavy crude totaled about 29,000 barrels a day last year for the entire Los Angeles area, NRDC scientists said.

Valero also wants to build a rail terminal to supply its refinery in the Bay Area community of Benicia with 70,000 barrels a day of petroleum products, including dirtier crudes such as tar sands.

Oil company officials say they are operating within state and federal regulations. As cleaner, liquid crude oil from California declines, they say they must rely on a variety of sources, including heavy Canadian crude, to remain profitable and ensure the future of their operations.

In an interview, Valero spokesman Bill Day said, "Valero follows the law. If we add more Canadian crude it will mean no net increase in emissions."

The request for an investigation, submitted to the South Coast Air Quality Management District, argues that "the highly corrosive nature of tar sands will increase the likelihood for spills and accidents, posing direct safety risks and increased toxic emissions for both plant workers and the surrounding community."

May said the sulfur found in heavy crude speeds corrosion in equipment and could lead to explosions like the one last summer at Chevron's refinery in Richmond. A Cal/OSHA investigation into the Aug. 6 explosion at the Bay Area refinery found that the company did not follow safety recommendations made by its inspectors to replace a pipe corroded by sulfur. The pipe ruptured and fueled the fire.

Environmentalists also worry that increases in carbon pollution will make it harder to meet requirements of the state's global warming law, AB 32, which created a market that puts a price on greenhouse gas emissions. Owners of power plants and factories buy and sell permits to release the gases into the atmosphere.

Mohsen Nazemi, deputy executive officer for the South Coast Air Quality Management District's office of engineering and compliance, said refineries are not obligated to report the sources of their oil products. "But we have, I believe, the most stringent rules and regulations in the nation when it comes to refineries." He said the refineries will not get any exemptions from regulations, regardless of the kind of crude they process.

Opponents see the local battle as part of a larger campaign against heavy Canadian crude that has stalled the Keystone XL pipeline project, which would carry tar sands oil to the Gulf of Mexico. As the Obama administration weighs approval of Keystone XL, a shortage of pipeline capacity has increased the use of oil trains bound for refineries here and across the U.S.

Last Wednesday, a train derailed in Minnesota, spilling 15,000 gallons of Canadian tar sands crude. On Friday, a ruptured Exxon Mobil pipeline spewed more than 12,000 barrels of the oil into the streets of Mayflower, Ark.

The issue has galvanized environmental activists in Wilmington, about 20 miles south of downtown Los Angeles in the heart of an industrial empire of cranes, cargo ships, chemical depots, rail yards, refineries and diesel-powered big rigs.

Alicia Rivera, a Wilmington resident and activist with Communities for a Better Environment, said, "For the oil companies, tar sands mean more profits, but for us it's a health issue.

"We're demanding that regulators measure the toxicity of tar sands oil and how it's affecting our community," she said. "And to make sure that happens, we're going to go door to door and hand out fliers that say, 'There's more pollution coming into Wilmington. Beware.'"